The recent drastic increase in the amount of signal information transmitted by a radio wave, which resulted from the remarkable improvement of information and telecommunication technologies, requires better accuracy and swiftness of the signal information and further increase in the amount of information, as well as has caused a rapid movement to the use of high-frequency radio waves, which in turn realized a full-scale use of a high-frequency band such as of 1 GHz or more, specifically of 10 to 20 GHz, which was not used before.
For example, for satellite communication, receiving and transmitting of radio waves are mainly performed by a parabola antenna with its antenna direction fixed, utilizing a stationary satellite. However, this transmission and receiving system can only handle a limited amount of information, which has caused a demand for a communication system capable of handling a larger amount of information. A recently established radio-wave communication system involves a lot of low-earth-orbit transfer satellites around the earth, through which radio waves are transmitted received, and a movable antenna located on the ground, which tracks the transfer satellites. This communication system has a superior advantage that every home user can transmit and receive a large amount of information through this system, which is comparable to an optical fiber.
As a lens antenna used in a movable antenna system for tracking low-earth-orbit transfer satellites, a Luneberg lens antenna (a dome type antenna, i.e., an antenna comprising a Luneberg lens) is used. This antenna has an advantage that it can handle radio waves coming from or transmitted to multiple directions simultaneously, and therefore is most suitable for a movable antenna system for tracking low-earth-orbit transfer satellites.
The Luneberg lens antenna includes a Luneberg lens which is capable of, for example, focusing radio waves. The lens used in this antenna is required to possess superior dielectric properties (e.g., high specific inductive capacity and low dielectric loss) so that it can handle a large amount of information, i.e. high-frequency radio waves, and to have a small dimension and low weight for workability and safety, since the antenna with this lens is installed on a roof of a domestic facility. For this, in Europe, the weight of a Luneberg lens antenna is officially standardized for installation works.
Meanwhile, various dielectric resin compositions as dielectric materials with good fabrication properties that are required for bulk production have been proposed, each comprising a synthetic resin and a dielectric inorganic filler, so as to cope with downsizing, weight saving and performance improvement of dielectric parts, which make up electronic devices such as an antenna, a condenser, a laminated circuit substrate, a connector and a memory.
For example, a dielectric resin composition comprising a synthetic resin and a powdery dielectric substance such as an alkaline earth metal titanate salt (e.g. barium titanate, strontium titanate, and barium strontium titanate) and the like has been known. A Luneberg lens using said dielectric resin composition has also been proposed (Japanese Patent Application Publication No. Sho-45-17242, Japanese Patent Application Publication No. Sho-56-17767). However, in order to obtain dielectric properties (high dielectric constant and low dielectric loss) that can cope with high-frequency radio waves by merely incorporating a powdery dielectric substance into a synthetic resin, a large amount of the powdery dielectric substance must be incorporated, which precludes those dielectric parts from having decreased weight and small dimension.
A dielectric resin composition comprising a synthetic resin and a fibrous alkaline earth metal titanate salt and the like has also been proposed (for example, Japanese Patent Application Laid-open No. Hei-5-299871). The fibrous dielectric substance can provide dielectric properties capable of handling high-frequency radio waves, even if it is contained in an amount less than that of the powdery dielectric substance, and therefore can save the weight thereof by some extent. On the other hand, the fibrous dielectric substance has such a property as to be oriented in a uniform direction in the synthetic resin and shows different dielectric properties in the substance between the orientation direction and the direction perpendicular thereto. This property is not a major problem in practical use of the existing electronic devices. However, in the progress of using higher-frequency radio waves and developments of electronic devices matching thereto, the dielectric parts which constitute said devices are required to have higher and more uniform dielectric properties, and therefore further improvements thereof are desired.
Furthermore, for example, Japanese Patent Laid-open No. Hei-8-133832 discloses a dielectric resin composition, in which a plate-like dielectric substance, which contains barium, titanium and rare earth elements as main components, is mixed in a synthetic resin. Since the plate-like dielectric substance, even if it is contained in an amount less than that of the powdery dielectric substance, can realize dielectric properties that can achieve a radio wave communication in a high frequency band, it contributes to weight saving by some extent in the same manner as done by the fibrous dielectric substance. However, the plate-like substance is also oriented in the synthetic resin, although the orientation does not extend to such a degree as the fibrous substance. This orientation of the substance may cause an ununiform dielectric profile, and therefore there still remains the necessity for improvement for the purpose of providing more uniform dielectric properties.
On the other hand, Japanese Patent Application Laid-open No. Hei-7-320537 discloses a dielectric polyurethane foam obtained by incorporating a powdery alkaline earth metal titanate into polyurethane and expanding the mixture. However, in order to enhance the dielectric characteristic of said foam to such an extent as to render a high-frequency radio wave usable, a large amount of powdery dielectric substance must be incorporated. This prevents downsizing and weight saving. Moreover, a large amount of powdery dielectric substance incorporated causes the mechanical strength of a resulting foam to be substantially decreased, which makes the foam impractical.